(1) Technical Field
This invention relates to electronic radio frequency (RF) circuits, and more particularly to RF phase shifter circuits.
(2) Background
Electronic phase shifter circuits are used to change the transmission phase angle of a signal, and are commonly used to phase shift RF signals. RF phase shifter circuits may be used for applications such as in-phase discriminators, beam forming networks, power dividers, linearization of power amplifiers, and phased array antennas, to name a few.
For many applications, it may be useful to serially-connect multiple phase shifter unit cells of the same or different phase shift values. Such phase shifter circuits may be digitally controlled and thus provide a discrete set of phase states that are selected by a binary control word, directly or after decoding. For example, such phase shifter circuits may be binary-coded, thermometer coded, or a hybrid combination of the two types. Some phase shifter circuits may also include a digitally controlled RF signal attenuator circuit that provides a discrete set of attenuation states that are selected by a binary control word, directly or after decoding.
FIG. 1 is a block diagram of a conventional phase shifter unit cell 100. Two ports, P1, P2, either of which may be an input port to the phase shifter cell 100 for an RF signal or an output port for the phase shifter cell 100, are coupled by a low pass filter (LPF) path 102 and a high pass filter (HPF) path 104. In the illustrated example, the LPF path 102 includes an LPF circuit 106, a primary LPF isolation switch 108, and an optional secondary LPF isolation switch 110 (the optional nature is symbolized by the dotted box). A common control signal, A, is coupled to the primary LPF isolation switch 108 and the optional secondary LPF isolation switch 110 (if present), and may be coupled to the LPF circuit 106 (the optional nature of such a coupling is symbolized by a dotted connection line). Similarly, the HPF path 104 includes an HPF circuit 112, a primary HPF isolation switch 114, and an optional secondary HPF isolation switch 116. A common control signal, Ā (i.e., the inverse or complement of the A control signal), is coupled to the primary HPF isolation switch 114 and the optional secondary HPF isolation switch 116 (if present), and may be coupled to the HPF circuit 112.
For small phase shifts, the LPF circuit 106 may be as simple as an inductor and the HPF circuit 112 may be as simple as a capacitor. For medium to large phase shifts, the LPF circuit 106 and the HPF circuit 112 may be more complex. For example, FIG. 2 is a schematic diagram of an LPF circuit 200 having a conventional Pi-type configuration. An inductor L1 provides a series connection between ports LP1, LP2, while bracketing capacitors C1, C2 connect ports LP1 and LP2 to circuit ground through corresponding FET switches M1, M2. In this example, the switches M1, M2 are controlled by the control signal A from FIG. 1, and help provide isolation when the LPF circuit 200 is to be isolated from ports P1 and P2. The operation of a Pi-type low pass filter is well-known in the art.
Similarly, FIG. 3 is a schematic diagram of an HPF circuit 300 having a conventional T-type configuration. Series-connected capacitors C3, C4 provide a series connection between ports HP1, HP2, while an interposed inductor L provides a connection from the junction of capacitors C3 and C4 to circuit ground through a corresponding FET switch M3. In this example, the switch M3 is controlled by the control signal Ā from FIG. 1, and helps provide isolation when the HPF circuit 300 is to be isolated from ports P1 and P2. The operation of a T-type high pass filter is well-known in the art.
Referring to FIG. 1, all of the isolation switches are typically implemented as field effect transistors (FETs) having a “CLOSED” or “ON” state (i.e., low impedance, signal conducting) and an “OPEN” or “OFF” state (i.e., high impedance, signal blocking) between the drain and source terminals, determined by a control signal to the gate of the FET. A conventional driver circuit (not shown) concurrently generates suitable voltages corresponding to the complementary control signals A and Ā for the isolation switch FETs and for the FETs within those embodiments of the LPF circuit 106 and the HPF circuit 112 that include switches (as in FIG. 2 and FIG. 3); such driver circuits are also known as level shifters. In some configurations, a bypass path (not shown) may be included that simply connects ports P1 and P2 through a FET switch while setting the LPF path 102 and the HPF path 104 to an isolation state.
In operation, the control signals A and Ā emanate from the same driver circuit and are complementary, meaning that they flip binary states in unison: when A=1, then Ā=0, and when A=0, then Ā=1. Accordingly, only one of the LPF path 102 and the HPF path 104 are coupled between ports P1 and P2 at any one time.
In particular, when the LPF path 102 is to be coupled between ports P1 and P2, then A=1 and Ā=0. Thus, the primary LPF isolation switch 108 and the optional secondary LPF isolation switch 110 (if present) are ON, as are any FET switches within the LPF circuit 106. Concurrently, the primary HPF isolation switch 114 and the optional secondary HPF isolation switch 116 (if present) are OFF, as are any FET switches within the HPF circuit 112.
Conversely, when the HPF path 104 is to be coupled between ports P1 and P2, then A=0 and Ā=1. Thus, the primary HPF isolation switch 114 and the optional secondary HPF isolation switch 116 (if present) are ON, as are any FET switches within the HPF circuit 112. Concurrently, the primary LPF isolation switch 108 and the optional secondary LPF isolation switch 110 (if present) are OFF, as are any FET switches within the LPF circuit 106.
For small phase shifts (e.g., less than about 12°), using a simple inductor for the LPF circuit 106 and a simple capacitor for the HPF circuit 112 provides good return loss (theoretically less than about 20 dB) and consumes little integrated circuit (IC) die area with few components and switches (in general, the optional secondary isolation switches 110, 116 are not needed).
For medium phase shifts (e.g., about 12° to about 90°), the more complex LPF circuit 200 and HPF circuit 300 of FIG. 2 and FIG. 3, respectively, may be necessary to maintain good return loss and achieve targeted phase shift states with realistic component values suitable for IC implementation. However, the optional secondary isolation switches 110, 116 generally would not be needed, thus saving IC die area.
For larger phase shifts (e.g., above about 90°), the more complex LPF circuit 200 and HPF circuit 300 of FIG. 2 and FIG. 3, respectively, are generally necessary to maintain good return loss and achieve targeted phase shift states with realistic component values suitable for IC implementation. In addition, the optional secondary isolation switches 110, 116 generally would be needed to provide adequate isolation.
For a system requiring a high resolution phase shifter, such as for RF domain cancellation or calibration, a high-isolation mode may be desired for a phase shifter unit cell 100 or a connected set of such cells. For example, in some applications, it may be a design criterion for a phase shifter to have a disabled, or all OFF state, so that the output port has 50 dB or more of isolation from the input port. A conventional solution would be to insert a FET switch in series with the input of the phase shifter chain—for example, at node X in FIG. 1—but doing so would introduce added insertion loss. More specifically, in RF circuits, the presence of a FET switch may have significant effects on the rest of the circuit, particularly with respect to termination impedance and isolation levels. Such effects arise because an “ON” (low impedance) FET has a non-zero resistance, RON, and an “OFF” (high impedance) FET behaves as a capacitor with capacitance COFF. Thus, adding a series FET switch to a phase shifter circuit will increase insertion loss due to the ON resistance, RON, of the FET while remaining capacitively coupled to external circuitry—and thus not fully isolated from the RF signal path—due to the capacitance COFF.
Accordingly, there is a need for a phase shifter unit cell or a connected set of such cells that can be well isolated from external circuitry and which does not introduce insertion loss into the RF signal path. The present invention addresses this need and provides additional advantages.